1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus using the same.
2. Description of the Background Art
Generally, an optical scanner for a laser printer or similar image forming apparatus includes various optical elements including an fθ lens. If the temperature of any one of the optical element varies, its optical characteristics vary due to thermal expansion with the result that scanning speed on an image surface and therefore the magnification of an image varies. This problem is particularly serious with a tandem image forming apparatus that forms a color image with a plurality of photoconductive elements and a plurality of fθ lenses assigned one-to-one to the photoconductive elements. More specifically, in this type of image forming apparatus, when the temperature of the individual optical elements arranged on optical paths varies, magnification varies from one element to another element in the main scanning direction due to thermal expansion, resulting in color shift. The temperature variation of, e.g., the fθ lenses is ascribable to hot air streams produced by a polygonal mirror, which spins at high speed, as well as other causes. The resulting thermal expansion causes the magnifications of images to differ from each other in the main scanning direction, so that colors are shifted from each other.
Japanese Patent Laid-Open Publication No. 08-146319, for example, discloses an image forming apparatus configured to sense changes in optical magnification and correct magnifications by varying pixel clocks or shifting mirrors on an optical path. Such correction, however, is difficult to execute page by page and is usually executed between consecutive jobs. It is extremely difficult to correct the variation of magnification each time during a series of jobs.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 10-186267.